1. Field of the Invention
The present invention relates to a semiconductor device which has circuits structured by thin film transistors (hereafter referred to as TFTs), and to a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment in which this type of electro-optical device is installed as a part.
Note that, in the specification, the term, semiconductor device, indicates a category of general devices which are capable of functioning by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic equipment are all included in the category of semiconductor devices.
2. Description of the Related Art
Development of a semiconductor device, which has a large surface area integrated circuit formed by thin film transistors (TFTs) structured by using semiconductor thin films (thickness on the order of several nm to several hundred nm) formed on a substrate with an insulating surface, has been advancing in recent years.
Active matrix liquid crystal modules, EL modules, and adhesion type image sensors are known as typical examples. In particular, TFTs using a silicon film with a crystalline structure (typically a polysilicon film) as an active layer (such TFTs are hereafter referred to as polysilicon TFTs) have a high electric field mobility, and therefore it is possible to use these TFTs to form circuits with various kinds of functionality.
For example, a display portion for performing image display every function block, and driver circuits for controlling the pixel portion and including CMOS circuits as shift register circuits, level shifter circuits, buffer circuits, and sampling circuits, are all formed on one substrate for liquid crystal modules installed in liquid crystal display devices.
Furthermore, a TFT (pixel TFT) is disposed in each of several hundred thousand to several million pixels in a pixel portion of an active matrix liquid crystal module, and a pixel electrode is formed in the pixel TFT. An opposing electrode is formed on an opposing substrate side across liquid crystals to form a kind of capacitor having the liquid crystals as dielectrics. There is a mechanism in which a voltage applied to each pixel is controlled by the switching function of the TFT and the liquid crystals are driven by controlling the electric charge to the capacitor, to control the amount of light transmitted and display an image.
The pixel TFT is composed of an n-channel TFT, which applies a voltage to the liquid crystals as a switching element, to drive the liquid crystals. The liquid crystals are driven by alternating current, and therefore a method referred to as frame inversion drive is often employed. In order to suppress electric power consumption with this method, it is necessary that the value of the off electric current of the pixel TFT (the drain current that flows when the TFT is in off operation) is sufficiently reduced.
Conventionally, a semiconductor film is instantaneously melted from the surface side in the case of irradiating laser light to the semiconductor film in order to increase crystallization. Then, heat transfer to a substrate then occurs, and the melted semiconductor film cools and solidifies from the substrate side. Recrystallization occurs during the solidification process to form a semiconductor film with a large grain size crystalline structure. There is temporary melting, however, and volume expansion occurs to form unevenness in the surface of the semiconductor film referred to as ridges. In particular, the surface with ridges in a top gate TFT becomes an interface with a gate insulating film, and thereby the element characteristics are greatly influenced.
In general, lasers often used in laser annealing are excimer lasers and Ar lasers. A method, in which a pulse emission laser beam with high output is processed by using an optical system on a surface to be a rectangular spot which is several cm square, or into a linear shape, for example, with a length equal to or greater than 10 cm, and then the irradiation position of the laser beam is scanned relatively to the surface to be irradiated, is preferably used because the high productivity and the superiority for mass production. In particular, if a laser beam with a linear shape (hereafter referred to as a linear shape beam) in the surface to be irradiated is used, the entire surface to be irradiated can be irradiated by scanning only in a direction perpendicular to the longitudinal direction of the linear laser beam, compared to a case of using a spot shape laser beam in which front and back, and left and right scanning is necessary. Productivity is thus high. Scanning in a direction perpendicular to the longitudinal direction is performed because the scanning direction has the highest efficiency. The use of linear shape beams, for which high output lasers are processed using suitable optical systems, in laser annealing is coming more and more into the mainstream due to their high productivity. Further, by irradiating the linear shape laser beam while gradually shifting in the short side direction and overlapping, laser annealing with respect to the entire surface of the amorphous silicon film can be performed. Crystallization can be performed, and crystallinity can be increased.
Laser annealing techniques are thus indispensable in manufacturing semiconductor films possessing higher electrical characteristics at lower cost.
However, a uniform amount of energy is not imparted over the entire film with crystallization performed by using conventional laser light and in addition to ridges, wave shaped traces where laser light has been irradiated also remain.